So Dwyer and his team turned to the Low Frequency Array (LOFAR), a network of thousands of small radio telescopes, mostly in the Netherlands. LOFAR usually looks at distant galaxies and exploding stars. But according to Dwyer, “it just works very well for measuring lightning.”
When thunderstorms are raging overhead, LOFAR can do some useful astronomy. So instead, the telescope adjusts its antennas to detect a barrage of about a million radio pulses emitted by each lightning bolt. Unlike visible light, radio pulses can pass through thick clouds.
The use of radio detectors to map flashes is not new; Specially built radio antennas have long observed storms in New Mexico. But these images are low resolution or only in two dimensions. LOFAR, the most modern astronomical telescope, can map lighting at a scale of meter by meter in three dimensions and with a frame rate 200 times faster than previous instruments. “LOFAR measurements give us the first really clear picture of what’s going on inside the thunderstorm,” Dwyer said.
Materializing lightning produces millions of radio pulses. To reconstruct a 3D image of lightning from the clutter of data, the researchers used an algorithm similar to that used in Apollo’s landings on the moon. The algorithm is constantly updating what is known about the position of the object. While one radio antenna can only indicate the rough direction of the flash, adding data from a second antenna updates the position. By continuously looping thousands of LOFAR antennas, the algorithm builds a clear map.
When researchers analyzed lightning data from August 2018, they saw that all radio pulses were emitted from a region 70 meters deep in the storm cloud. They quickly concluded that the pulse model supports one of two leading theories about how the most common type of lightning begins.
One idea is that cosmic rays – particles from space – collide with electrons in thunderstorms, triggering electronic avalanches that strengthen electric fields.
New observations point to competition theory. It starts with clusters of ice crystals inside the cloud. Turbulent collisions between needle crystals remove some of their electrons, leaving one end of each ice crystal positively charged and the other negatively charged. The positive end draws electrons from nearby air molecules. More electrons are infused by air molecules that are farther away, forming bands of ionized air that extend from each tip of the ice crystal. They are called streamers.
Each crystal peak gives rise to hordes of streamers, with individual streamers branching out over and over again. Streamers heat the ambient air, extracting electrons from the air molecules en masse, so that more current flows over the ice crystals. Eventually, the streamer becomes hot and conductive enough to become a leader – a channel through which a full strip of lightning can suddenly move.
“This is what we see,” said Christopher Sterpka, the first author of the new document. In a film showing the initiation of the flash, which the researchers made from the data, the radio pulses increased exponentially, probably due to the flood of streamers. “Once the avalanche stops, we see a lightning guide nearby,” he said. In recent months, Sterpka has compiled more lightning-fast films that look like the first one.